The present invention relates to the optical inspection of glassware and like transparent articles, and more particularly to inspection of transparent articles for defects during translation or rotation past an optical inspection station.
It is well known to provide automated inspection of glassware and like transparent articles by illuminating the article and optically monitoring light transmitted, refracted, or reflected by the article. Various such techniques rely upon a total or partial occlusion of light rays due to a defect in the article, which is detected as a darker-than-normal spot in the image. Alternatively, light may be reflected by a defect toward the photosensor, resulting in a brighter-than-normal image area. Signal processing circuitry is typically employed to amplify and shape the photosensor output, for comparison to preselected threshold values indicative of faults. These techniques have typically suffered "false positives" and "false negatives" due to random light patterns, container surface characteristics, artifacts, crosstalk in the inspection electronics, and other illusory effects.
One factor of critical significance in the operation of such inspection systems is the motion, if any, which the article may undergo during the inspection period. The usual approach has been to control article motion during inspection to provide an angular displacement over time between the article and the photosensor. Specialized conveyor apparatus may be required for this purpose.
U.S. Pat. No. 4,256,957 to Ford et al. discloses bottle inspection apparatus wherein an image of a bottle area is projected onto an array of photodiodes to derive a video signal based upon a sequence of light energy readings from individual pixels. This signal is differentiated to provide a "differentiated frame video signal" representative of the outputs of successive whole row of photodiodes. The Ford et al. device permits the inspection of bottles travelling on high speed conveyors by limiting the inspection interval to around 1 millisecond. As such, their system is unsuitable for "real time" inspection of glassware containers, for example while rotating the container to scan the entire circumference. This system will therefore be vulnerable to illusions which might lead to invalid data at a given scanning configuration.
U.S. Pat. No. 4,280,624 to Ford teaches apparatus for inspecting the sidewalls of bottles in which a plurality of sidewall images are projected from two or more different directions onto one or more photodiode arrays, thereby to scan the entire bottle circumference. This approach suffers the shortcoming mentioned above of vulnerability to illusory readings which may occur in a given bottle position or orientation.
Commonly assigned U.S. Pat. No. 4,488,648 to M. Claypool relies upon an optically generated signal which is processed in real time by repetitively calculating the time rate of change and comparing these calculations to preselected values as a basis for fault rejection. This technique represents a useful advance over the prior art, but requires relatively complicated preliminary analysis as a prerequisite to inspection.
U.S. Pat. No. 4,492,476 to Miyazawa discloses a defect inspection technique for bottles wherein the bottle is conveyed while being spun, and video images thereof are acquired sequentially and repeatedly in the form of a plurality of picture elements. Picture element signals on an imaginary line perpendicular to the central axis of the image are compared, and the results of this comparison are processed to discriminate defects. This technique does not achieve a principal advantage of applicants' technique of effectively reducing the likelihood of falsely identifying spurious signals, random light patterns, etc. as defects.
Accordingly, it is a principal objective of the invention to provide improved method and apparatus for automated optical inspection of glassware articles and the like. A related object is to provide a reliable technique for identifying features of an article which is moving relative to a photoinspection device.
A further object is to effectively discriminate between features of interest in such articles and illusory phenomena, such as random light patterns, artifacts, and camera electronics crosstalk.
A still further object is to adapt this technique to the detection of brighter-than-ambient image features, as well as darker-than-ambient features.